1. Field of the Invention
The invention relates to an antenna amplifier and a shared antenna amplifier which amplify a weak signal received by an antenna, at a position in the vicinity of the antenna.
2. Description of the Related Art
FIG. 20 schematically shows the electrical configuration of a conventional antenna amplifier 1. The antenna amplifier 1 is used for amplifying a weak high-frequency signal which is induced as a result of reception of a radio wave by an antenna, at a position in the vicinity of the antenna to enhance the signal-to-noise (SN) ratio. The weak high-frequency signal is amplified by a low noise amplifier 2. A band-pass filter (hereinafter, abbreviated to “BPF”) 3 which limits the frequency band, and an attenuator (hereinafter, abbreviated to “ATT”) circuit 4 are disposed on the side of the input of the amplifier 2. When the output of the amplifier 2 is increased, an automatic gain control (hereinafter, abbreviated to “AGC”) circuit 5 lowers the signal attenuation factor of the ATT circuit 4 to lower the whole gain of the antenna amplifier 1. The antenna amplifier 1 has a further effect that a signal cable connecting the output of the amplifier 2 with a receiving apparatus such as a tuner is prevented from affecting the antenna.
Japanese Unexamined Patent Publication JP-A 5-102874 (1993) discloses a radio receiver antenna apparatus into which an RF preamplifier is incorporated. In the apparatus, bypass circuit means is disposed so that an RF signal is detected also in a path other than the RF preamplifier, and, when a strong signal is received by an antenna, the RF preamplifier is bypassed to prevent the amplifier from being overloaded. FIG. 2 of the publication shows a configuration in which a bypass circuit is normally connected to the preamplifier, and, when the RF preamplifier is to be bypassed, paths for input/output signals to the base and collector of a transistor that is used for amplification in the RF preamplifier are interrupted.
A technique relating to disposition of an amplifier circuit between an antenna and a tuner is disclosed also in, for example, Japanese Unexamined Patent Publication JP-A 11-41127 (1999). In the technique, a parallel circuit of a broadband amplifier and an attenuation circuit is connected between an antenna and a tuner, and a power supply to the broadband amplifier is controlled or switched so that, when the broadband amplifier is operated, a signal is amplified by the amplifier, and, when the broadband amplifier is not operated, a signal is attenuated by the attenuation circuit. Japanese Unexamined Patent Publication JP-A 7-321685 (1995) discloses a technique in which an amplifier and an attenuator are switched over by an amplifier circuit for an intermediate frequency that is converted from a high frequency by a front end for a tuner, so that the electric field strength can be correctly detected while avoiding a situation where the amplifier is saturated at high field strength.
Another reason of disposition of the AGC circuit 5 in FIG. 20 is to prevent the amplifier 2 from being distorted at high field strength. Japanese Unexamined Patent Publication JP-A 11-145858 (1999) discloses a technique relating to an AGC in which the gain is lowered only when sensitivity deterioration due to intermodulation interference occurs. Japanese Unexamined Patent Publication JP-A 11-312938 (1999) discloses a technique in which a time constant of an AGC is changed in accordance with the field strength in such a manner that the time constant is normally reduced so as to easily follow a fading phenomenon, and a time constant is increased in an environment of high field strength, thereby improving the stability.
In the antenna amplifier 1 shown in FIG. 20, in a case such as that where another signal of an excessive level exists at a frequency which is in the vicinity of the frequency of the target received signal, even when the ATT circuit 4 is switched so as to attenuate the signal, there is the possibility that the input level of the amplifier is excessively high and the target received signal is distorted or suppressed because of the nonlinearity of the amplifier 2. When the attenuation factor of the ATT circuit 4 is set to a large value, also the target signal is largely attenuated, and the SN ratio is lowered.
In the technique disclosed in JP-A 5-102874, when the RF preamplifier is bypassed, the operation of the RF preamplifier is completely stopped.
In the technique disclosed in JP-A 11-41127, the position where the amplifier circuit is to be disposed is not particularly clearly indicated. In the paragraph of “Embodiment of the Invention” of the publication, the whole set of the broadband amplifier, the attenuation circuit, and the tuner is defined as a tuner circuit. Therefore, it seems that the amplifier circuit is disposed not in the vicinity of the antenna, but on the side of the tuner. When it is assumed that the amplifier circuit is disposed as an antenna amplifier at a position which is separated from the tuner and in the vicinity of the antenna, a power supply cable which is ON/OFF controlled must be connected together with a signal cable between the antenna amplifier and the tuner. The power supply is controlled on the basis of the signal level of the stage subsequent to the tuner. The signal level is affected by the ON/OFF control of the broadband amplifier. Specifically, there is the possibility the following is repeatedly performed. When the signal level is judged to be excessive during the ON state of the broadband amplifier, the broadband amplifier is set to the OFF state, and the signal level is lowered. As a result, the signal level is judged not to be excessive, and the broadband amplifier is again set to the ON state.
In the case where an antenna amplifier is to perform an AGC operation, there arise problems of distortion and signal suppression due to intermodulation or cross modulation which occurs when an interference signal exists in the vicinity of a desired reception frequency. No countermeasure against such a case is described in JP-A 11-145858 and JP-A 11-312938. When an antenna element is to be used for a broad band, the signal range in which intermodulation or cross modulation may occur is widened. In a shared antenna amplifier which is used for sharing an antenna element in a broad frequency band, therefore, an adequate countermeasure must be taken.